Ramp analog-digital converters use
a comparator receiving a reference voltage and a useful voltage with the aim of converting the difference between the useful voltage and the reference voltage,
a rising or falling linear voltage ramp, which is superimposed on one of the voltages from a given instant,
a digital counter that counts pulses at a fixed rate from the start of the ramp;
a memory that preserves the content reached by the counter at the instant at which the output state of the comparator toggles on account of the voltage variation caused by the ramp and applied to the comparator.
The content of the counter is a digital representation of the difference between the useful voltage and the reference voltage.
In an image sensor, the reference voltage may be a voltage sampled on a column conductor after a reset operation for a pixel, and the useful voltage is the voltage sampled on this same column conductor in a read operation for the charges generated in the pixel by the illumination.
For various reasons that will be returned to later, there may be a requirement to convert two useful voltages and to take the average of the results of the conversion. To give a quick example, there may be a need to simultaneously convert the signals from two adjacent pixels in a matrix sensor and to obtain the average of the results; the values provided by two adjacent pixels when there is not a lot of light and a loss of image resolution is accepted are grouped together. In another example, there may be a need to reduce reading noise for the pixels by taking two successive samples of the voltage to be converted, by converting these two voltages and by taking the average of the results of the conversion.
In these two examples, the solution involves performing two (simultaneous or otherwise) analog-digital conversions and digitally adding the results of the two conversions. However, this solution requires processing of digital values, which is ponderous and cumbersome in an integrated circuit, for example when this processing needs to be performed on hundreds of columns of a matrix.